Apparatus for generating radiation of frequencies higher than those of light



June 11, 1968 G. GOULD 3,3

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APPARATUS FOR GENERATING RADIATION OF FREQUENCIES HIGHER THAN THOSE OF LIGHT Filed Aug. 21, 1967 8 Sheets-Sheet 7 Tijl.

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nited States Patent 3,388,314 APPARATUS FOR GENERATEG RADIATION 0F FREQUENCIES HEGHER THAN THOSE 3F EIGHT Gordon Gould, New York, N.Y., assignor, by mesne assignments, to Control Data Corporation, South Minneapolis, Minn, a corporation of Minnesota Original application Apr. 6, 1959, Ser. No. 804,544). Divided and this application Aug. 21, 1967, Ser. No. 665,667

3 Claims. ((31. 321--69) AESTRAT 0F THE DISCL'OSURE Apparatus of various types is disclosed for the amplification and generation of light by the stimulated emission of photons from ions, atoms and molecules in the gaseous liquid and solid state with the amplified light being characterized by spatial or time coherence, including types of apparatus in which the ions, atoms or molecules are excited by light or by an electronic discharge and types of apparatus in which the stimulated emission of photons is accomplished in a resonator consisting of a pair of op posed specular reflectors or other environment Where the light energy is caused to build up in intensity; and systems incorporating such light amplifying or generating apparatus, including systems for the generation of intense heat and high temperature reactions in solids, liquids and gases, communications systems and distance and time measuring systems.

This application is a division of application Ser. No. 804,540, filed Apr. 6, 1959.

The present invention relates to light amplifying devices operating on principles similar to those utilized in a maser (microwave amplification by stimulated emission of radiation) and to various systems incorporating such devices. More particularly, the invention relates to the amplification of light by the stimulated emission of photons from ions, atoms, or molecules in gaseous, liquid or solid state; in particular cases this is accomplished in a Cavity or other environment where the light energy will not be free to escape unimpeded, but may be caused to build up in intensity. The amplified light in such apparatus is characterized by spatial or time coherence.

A short explanation of the physical principles involved will be helpful in eXplaining the nature of the invention.

It is known that atoms, ions or molecules (hereinafter called molecules) ordinarily exist in so-called stationary states possessing a more or less well defined energy. While in such a state a molecule does not exhibit an oscillating electric or magnetic moment. However, since a molecule is made up of charged particles, it will be perturbed by any oscillating electric or magnetic field in which it may lie. When so perturbed, a molecule originally known to be definitely in stationary state a will possess a certain probability of being found in state b with different energy. When in such a mixed state, the molecule may exhibit an oscillating electric or magnetic moment (i.e., it may appear as a system of oscillating charges, or charges in changing orbits). A molecule will undergo a transition from state a to state b (i.e., have a large probability of being in state b) if the induced electric or magnetic moment oscillates with almost the same frequency as the applied electric or magnetic field, and if the polarizations and phases of the oscillations correspond. The frequency of the oscillating moment is determined by the Einstein relationship:

AE: h 11 where a 3,388,314 Patented June 11, 1968 v =tl1e oscillation frequency h Plancks constant A .=the energy difference between the two molecular states.

The same equation E=hv gives the energy of the photons associated with the electro-magnetic field. The photon density is proportional to the energy density of the field. During a transition, a photon or quantum electromagnetic energy is emitted to or absorbed from the field, depending on whether the molecule is changing from a higher to lower energy state or vice-versa.

Even when there is no radiation energy density of the right frequency directly observable at the molecule, spontaneous transitions occur from higher to lower states with the emission of photons. These transitions are actually induced by random fluctuations in the electro-magnetic field of so-called empty space.

The photons emitted during an induced transition have the same phase and polarization as the inducing wave i.e., they are coherent with it. A single atom may radiate a photon in any direction. However many atoms distributed over a finite volume and radiating coherently cooperate to generate a wave having the same propagation vector as the inducing wave, within the limits of a diffraction pattern. That is, they amplify the inducing Wave. The radiation from induced emission has a spectral distribution similar to that of the inducing radiation and may be in a very sharp line.

Spontaneously emitted photons, because of the random nature of the zero-point fluctuations, have no definite phase or polarization. Because the zero-point fluctuations contain all frequencies, spontaneously emitted radiation has a finite bandwidth, characterized, at the least, by a Lorentzian line shape.

In thermal equilibrium, the populations of two states are related by the Boltzmann distribution factor:

EhizhElou Nam T] low Thus, in equilibrium the population of a higher energy state is less than that of a lower energy state. In particular, the population of a state separated from the lowest by an optical frequency is practically nil at ordinary temperatures. Induced transitions under these conditions necessarily absorb photons from the radiation field.

The foregoing principles can be utilized to devise an paratus for microwave amplification by stimulated emission of radiation which has been termed a maser. If by some means the population of a higher energy state is made larger than that of a lower energy state, induced transitions must necessarily result in the emission of photons to the radiation field. Thus a molecule may emit spontaneously a photon which in turn may induce coherent emissions in neighboring molecules, adding to the total radiation energy. If the transition is at a microwave frequency, the system may be enclosed in a cavity resonant at the same frequency and the escape of the photons prevented. If the power loss from the cavity is less than the power emitted from the molecules, the system will oscillate with a frequency which fluctuates much less than the (Lorentz) bandwidth of the transition. The condition for maser oscillation in a gas is that the excess population density It h i) Z of the gas fills the cavity. T=T1=Tz is the relaxation time or state lifetime. QEthe quality factor of the cavity. p is the oscillating electric or magnetic moment characterizing the transition.

If the condition for oscillation is not quite met but external power is coupled into the cavity, the sensitized or pumped molecules will add to or amplify the signal. Because power is lost through the output coupling, the condition for infinite gain, at optimum output, is

h b ---Ni) 2 W This amplification adds -very little random noise to the amplified signal. The minimum noise is determined by thermal fluctuations in the radiation field or by random spontaneous emission, whichever is larger.

Several methods have been proposed for maintaining an excess population in the higher of two molecular energy states of a gas filling a resonant maser cavity. One form of maser which has been proposed achieves optical pumping by unpolarized light.

The discussion of this form of optical pumping will be given in terms of rubidium (atoms) but would be similar for other cases. Light, characteristic of various spontaneous transitions in Rb, is generated in a discharge lamp and passed through a filter. The filter removes all frequencies except that component line which induces tran sitions from the F=1 hyperfine level of the ground electronic level to some particular higher electronic level.

Spontaneous decays back to both hyperfine ground levels will result in a net pumping of Rb atoms from F:l to F=2.

To maintain an excess population in F=2 over F: l, the optical pumping rate need only exceed the collision relaxation rate which may be made as small as 10/ sec. Of course this minimum pumping rate would give a cor respondingly small power output from the maser.

In the light amplifier, on the other hand, the negligible thermal population of higher electronic states and the high rate of spontaneous emission from these states, make nec essary a much higher pumping rate. In general, these effects preclude light amplifier operation between a higher state and a ground state. Usually, a transition between two higher electronic states must be utilized.

Like the maser, the light amplifier will operate on the principle of induced transitions from a higher energy state to a lower energy state with smaller population. However, the techniques usable and possible are appropriate to the optical region of the electro-magnetic spectrum. This fi'equency range is defined for the present purpose by the limit of transparency of materials in the infrared and ultraviolet to be approximately:

lcm. cm. 3X10 cycles/ sec. v 3 10 cycles/ sec. in practice Another limitation which becomes serious in the far ultraviolet is the amount of power spontaneously emitted by the active atoms. This emitted power increases as 11 and must be equaled or exceeded by the input power in order to have light amplification. At A: 1000 A. an input power of the order of l kilowatt is required. Below this wavelength the required input power is too large to dissipate.

Likewise, the useful properties of the light amplifier are qualitatively different from the maser and derive from the vastly shorter wavelength and higher frequency of the radiation involved.

The previous explanation has been given in terms of amplifiers and amplification, but it should be understood that if suflicient gain can be achieved, the light amplifying apparatus can be rendered self-oscillating. Thus a controlled light oscillator may be provided as well as an amplifier. The systems utilizing the apparatus as an oscillator will also be useful.

One form of the light amplifier which will be described as the non-resonant form may be utilized as an oscillator to generate light waves which fall within a narrow frequency bandwidth and have an exceedingly constant average frequency. Light from such an oscillator, particularly when short term frequency perturbations have been 4 effectively eliminated by averaging over a finite period of time, provides a light frequency standard having an accuracy comparable to the accuracy of any known frequency standards. A light source of this type has obvious applications in the measurement of time, frequency, and particularly of distance by interferometric techniques.

Another form of light amplifier according to the present invention, which will be described as a resonant form, has somewhat different characteristics and is useful for different and quite varied applications. It is a characteristic of a resonant light amplifier constructed according to the present invention that its output is in the form of a beam which is very nearly a plane wave if the input is a plane wave. That is, the divergence of the beam may be very small so the energy of the beam is substantially contained within a very small solid angle of the order of l0 steradians or less.

Apparatus according to the present invention provides the capability of amplifying light coherently, at least with respect to frequency and phase and in some cases also with respect to direction of propagation, and aXis of polarization. Insofar as is known no apparatus with this capability has heretofore been produced.

A coherent infrared light molecular amplifier and generator has been proposed in U.S. Patent No. 2,851,652 to Robert H. Dick. This apparatus and methods proposed by Dicke dilfer in many respects from those of the present invention. In the Dicke device ammonia molecules are provided in a bounded volume wherein the higher of two molecular energy states is more highly populated than the lower; this is accomplished by physically separating, by electric fields, the molecules in the lower state from a beam of molecules before the beam is permitted to enter the bounded volume. Such activated molecules are capable of amplifying an electro-magnetic wave within a particular frequency range.

The present invention, on the other hand, causes the atoms, ions or molecules of the working medium to be activated to produce the desired population excess in the higher energy level without actual physical separation. Furthermore, the desired population excess is produced within the bounded volume in which emission takes place in the present invention; whereas, in the Dicke apparatus the physical separation to produce the desired population distribution is accomplished outside the cavity after which the working medium is physically transported into the bounded volume in which stimulated emission takes place.

A simple very effective apparatus results from the different method and apparatus for activation utilized in the present invention.

Resonant oscillators according to the present invention have the characteristic of nearly plane wave output. Accordingly, a unique communication system can be provided by the present invention. The capabilities of such a system can be appreciated by the fact that the output light energy may be concentrated within a solid angle of the order of 10- steradians; this is well within the capabilities of such an oscillator. Such a beam need only be modulated with any information which it is desired to transmit, and detected at the receiving station at a distance (limited by line of sight considerations and atmospheric transmissibility) by conventional photoelectric or other suitable techniques and demodulated. Communication systems utilizing such apparatus would have advantages over virtually all known systems including microwave radio transmission systems and the like, from the point of view of efliciency and amount of information transmitted per channel.

A further system utilizing the principles of the invention makes use of the near planarity of the light wave from a resonant oscillator. When combined with a focusing means such as a parabolic mirror, the resonant oscillator provides a means for concentrating considerable energy in a volume much smaller than had heretofore been possible thus allowing the creation of very high temperatures within a very small volume. In addition to use for scientific research, such a heat source may be used in high temperature processes for industrial purposes and in other applications where exceedingly high temperatures are required.

Although various specific systems incorporating the light amplifier or oscillator have been mentioned above and will be described in more detail, there are numerous other applications of the light amplifying device such as projection TV, high speed photography, precision measurement of length and velocity, illumination, control systerm for various kinds of equipment, generation of X-rays, etc. In addition to the above objects and advantages of the present invention, it is an object of the present invention to provide a light arnplifier wherein light is arnplified by raising atoms, ions or molecules to a particular energy level and stimulating them by exposure to light having a frequency corresponding to the energy difference between that level and a lower energy state of the atoms, ions and molecules thus stimulating the atoms, ions or molecules to decay to the lower energy state with the emission of light energy coherent with the stimulating light, thereby amplifying it.

It is another object of the present invention to provide such a light amplifier wherein a confined space is provided I with reflecting walls causing emitted light to re-traverse said space repeatedly causing additional stimulation of atoms, ions or molecules within said space.

It is still another object of the present invention to provide such a light amplifier utilizing specular reflectors spaced so that the paths therebetween are of a definite distance representing a definite number of wavelengths of the amplified light whereby other wavelengths are discriminated against and whereby a nearly planar wave of light is generated.

It is still another object of the present invention to provide such a light amplifier wherein the conditions for oscillation may suddenly be changed, thereby causing the excited atoms, ions or molecules to emit photons in a short, transient wave-train or light pulse of greater peak power than may be generated in steady-state operation.

It is a further object of the present invention to provide such a light amplifier wherein the atoms, ions or molecules are raised by exposure to light from a gaseous discharge lamp to a particular desired energy level (from which stimulated decay is to be produced).

It is a still further object of the present invention to provide a light amplifier of the type described immediately above in which the gaseous discharge lamp is filled with an element different from that of the atoms, ions or molecules to be excited and which has an atomic resonance which by change coincides with a spectrum line of the atoms, ions or molecules to be excited.

It is a still further object of the present invention to provide such a light amplifier wherein the atoms, ions or molecules are raised to a particular desired energy level (from which stimulated decay to a lower level is to be produced) by supplying electrical energy to produce a discharge within the confined space in which the atoms, ions or molecules to be excited are contained.

It is a still further object of the present invention to provide such a light amplifier wherein the atoms, ions or molecules are raised to a particular desired energy level (from which stimulated decay to a lower level is to be produced) by utilizing collisions of the second kind with atoms, ions or molecules of a different element or elements from that to be stimulated to produce the emission of light.

It is a still further object of the present invention to provide a light amplifier having an optical system with reflecting elements having surfaces subtending predetermined angles by which light from a range of angles is accurately reflected back upon itself in a manner analogous to the operation of a microwave corner reflector.

It is an additional object of the present invention to provide such a light amplifier having an optical system consisting of filters and reflectors which will permit the generation light only Within a predetermined frequency range and with a definite polarization.

Other objects and advantages will be apparent from a consideration of the following description in conjunction with the appended drawings, in which:

FIG. 1 is a partially schematic illustration in crosssection of a nonresonant light amplifier designed to be excited by an external source of light radiation;

FIG. 2 is a Grotrian diagram of energy levels of sodium presented to aid in the explanation of light amplifying apparatus according to the present invention;

FIG. 3 is a partially schematic illustration in cross section of a nonresonant light amplifier designed to be excited by an electrical discharge within the amplifier cavs;

FIG. 4 is a partially schematic illustration of a resonant light amplifier excited by incoherent light radiation and utilizing triangular prisms as reflecting surfaces;

FIG. 5 is a partially schematic illustration in crosssection of an alternative optical system for the apparatus of FIG. 4, for example, in which only light polarized in the plane of the paper will be generated;

FIG. 6 is a partially schematic illustration in crosssection of a resonant light amplifier excited by incoherent light radiation and utilizing optically flat parallel mirrors for its reflecting surfaces;

FIG. 7 is a partially schematic illustration in cross-section of a resonant light amplifier designed to be excited by a discharge within the resonant cavity;

FIG. 8 is a diagram of the energy levels of the iodine molecule useful in explaining a form of the invention utilizing coincidence of spectral lines in energizing a working medium in a light amplifier;

FIGURE 9 is a diagram of the energy levels of the Europiurn ion, Eu+++, useful in describing a form of the invention utilizing a non-gaseous working medium;

FIG. 10 is a schematic diagram of a communication system incorporating resonant light amplifiers and oscillators;

FIG. 11 is a schematic diagram of a high temperature heating apparatus utilizing a resonant light oscillator according the present invention;

FIG. 12 is a schematic diagram of a frequency stand ard employing nonresonant light oscillators according to the present invention;

FIG. 13 is a schematic diagram of interferometric apparatus comprising a nonresonant light oscillator according to the present invention;

FIG. 14 is a diagram of some energy levels of z nc useful in describing a form of the invention utilizing an internal discharge for exciting the working medium;

FIG. 15 is a partially schematic illustration in crosssection of a form of resonant light amplifier wherein the light path traverses a Kerr cell by means of which the losses of the system may be rapidly varied in order to generate a transient light pulse.

516. 16 is a partially schematic diagram of a system for generating short light pulses comprising a pulsed resonant light amplifier of the type shown in FIG. 15, for example, a rapid shutter for trimming the ends of the transient wave-train, and a nonresonant light amplifying tube which further shortens and peaks the pulse;

FIG. 17 is a partially schematic diagram of a system Wheerin the light beam from a resonant light amplifier is manipulated by mirrors to scan an object and by detection of the reflection therefrom to provide information for the automatic control of equipment which may comprise a pulsed light amplifier and moving mirrors;

FIG. 18 is a partially schematic diagram of an evaporative machining apparatus utilizing a light oscillator according to the present invention; and

FIG. 19 is a partially schematic diagram of apparatus for subjecting a liquid to a very high temperature and ;lil,388,314

utilizing a light oscillator according to the present invention.

Nonresorzant light amplifier Referring to FIG. 1, there is shown at 11 a spherical cavity 11 forming a principal part of a nonresonant light amplifier. Although a spherical cavity has an optimum volume to surface ratio, the cavity need not be of this shape but could be cylindrical, rectilinear, or of other shape if desired.

The cavity 11 is provided with apertures 12 and 13 for the output and the exciting light input respectively for the cavity. Suitable windows 14 and 15 are provided to cover the apertures 12 and 13 and should be made of a material such as glass or the like, having a high transmission coeflicient for the frequency of light involved.

The interior 16 of the cavity 11 is filled with a sensitized working medium in this form of the invention, the nature and function of which will be hereinafter explained in more detail.

The wall 17 of the cavity 11 is rendered reflective as by i a reflective coating 20. This surface may either be a specular reflector such as polished metal, or a difiuse reflector. For light in the visible region the highest reflectivity is achieved with a diifuse reflector such as magnesium oxide powder, and such a reflective surface would generally be preferred for the reflecting coating 20.

A gaseous atmosphere for the cavity interior 16 is supplied from a reservoir 18 connected to the cavity by a conduit 19.

A heating coil 21 controlled by a temperature regulator 22 may be utilized to maintain the vapor in the interior of the cavity at the desired pressure. A temperature control oven 23 is provided surrounding the cavity 11 to mantain the cavity at a temperature higher than that of the reservoir 18 thus preventing condensation in the cavity 11 and assuring control of pressure by means of a temperature regulator 22 regardless of changes in ambient temperature of the cavity.

Light is directed from an exciting light source 24 through the window 15 to the interior of the cavity 11. In a typical case, the light source 24 will comprise a gas discharge lamp having a gaseous atmosphere similar in composition to that of the atmosphere in the interior 16 of the cavity 11.

Although solid or liquid fluorescent material may be advantageous in certain applications rather than a gaseous medium within the cavity 11 in FIG. 1, transition processes in gases are more completely understood and accordingly the explanation will be primarily directed to this more readily understood form.

Operation of nonresonant light amplifier The operation of the light amplifier of FIG. 1 will first be explained with reference to a relatively simple form of excitation, that is, excitation by resonance radiation. It

should be understood that other forms of excitation which will later be explained may in many cases be preferable to the simpler type of excitation by resonance radiation.

A desirable medium for this form of excitation is sodium vapor and for the purpose of this explanation, it will be assumed that the interior 16 of the cavity is filled with sodium vapor and that the exciting light source 24 is a sodium vapor lamp.

FIG. 2 is a diagram of some of the higher electronic levels of sodium. The hyperfine structures of these electronic levels are not shown.

The free-space wavelengths (in Angstroms) of the electro-magnetic radiation emitted during transitions between certain pairs of levels are given on the diagram of FIG. 2. The measured or estimated spontaneous emission rates for these transistions are also indicated. Electric dipole radiation selection rules permit transistions only be tween levels in adjacent columns of the diagram, Thus no transitions occur between levels with the same letter designation (same orbital angular momentum). Atoms in the ground level (3 8 can be excited by resonance radiation from a sodium lamp only to the various P- levels. However, all states may be excited by collisions with energetic electrons in a discharge or by collisions with other excited atoms (collisions of the second kind).

It is desired to achieve a higher population in some higher level than in a lower level, to whichtransitions may be induced by the presence of light energy of suitable frequency. If it is assumed that only the 6 F levels are excited directly from the ground level, light exciting other levels could be removed by an appropriate optical filter. Then by spontaneous emission various lower levels will become populated to some extent. The populations in dynamic equilibrium may be calculated from the spontaneous decay rates.

If the 6 levels are assumed to have a unit population, the computed populations of the other lower levels are shown in the diagram. It may be noted that the population of the 4 8 level is only 0.0067 of the 6 F levels population and hence transistions generating the 8660 A. infrared line may be expected in a suitable enclosure.

From the diagram of FIG. 2 and the foregoing explanation, it will be seen that when the medium in the cavity 11 is excited by the light from the light source 24, a condition is produced where the population of a higher energy level (6 F) is much higher than the population of a lower energy level (4 8 so that the presence of light of the frequency represented by the difference between these two energy levels (wavelength 8660 Angstroms) will stimulate decay from the higher energy level to the lower energy level with the emission of more light of this same frequency.

Accordingly, when the pumping rate due to excitation from the source 24- is sufilciently great to maintain a large population diiierence between these two levels in favor of the higher level, and when losses in the cavity are reduced to a sufficiently low level as by maximizing the reflectivity of the surface 20, conditions for sustained oscillation will be met and the apparatus of FIG. 1 will operate as a nonresonant light oscillator.

Obviously, if the conditions for oscillations are approached but are not met, light of the appropriate fre quency (18660 Angstroms) introduced into the cavity will be amplified by the stimulated emission of radiations and the output of the cavity at that frequency will be greater than the input thus providing amplification, but self-sustained oscillation will not occur.

The nonresonant light amplifier of FIG. 1 is schemati cally shown with a relatively small window for the introduction of light excitation; in practice, it will generally be desirable to utilize a substantial portion of the surface of the cavity as a window for light excitation. It will be recognized that increasing window area cuts down on the available surface for reflection. The effective reflection coeflicient may be kept relatively high by arranging the reflective portions of the cavity on opposite portions of the surface of the amplifier enclosure.

If the window area desired for light excitation is a substantial portion of the total area of the cavity, it may be preferred to make the amplifier in another form, such as cylindrical, for example. This form may be particularly desirable as the curved peripheral surface of the cylinder may be made transparent for the introduction of light excitation while the ends of the cylinder may be rendered diffusely reflective. With this arrangement a large amount of light power may conveniently be transmitted into the cavity. Although there may be some rednction of average effective reflection coefficient, this is offset by other considerations.

If the cylindrical nonresonant amplifier described above is made in elongated form, only light within a narrow angular range of direction of propagation will be amplified and thus the noise due to spontaneous emission will be reduced, yielding a narrower output bandwidth. The

output of an elongated cylindrical nonresonant amplifier may largely be restricted to an angle on the order of approximately 6; this is much more convenient and may be directed more efiiciently than the diffuse output as from a spherical amplifier or oscillator.

Tendency toward resonance in the elongated cylindrical nonresonant light amplifier will be avoided by the fact that light paths of many difierent lengths will exist between the reflectors; if desired, the reflectors may be shaped to increase the diversity of optical path lengths between reflectors.

Various elements other than sodium may be utilized in the construction of such a nonresonant amplifier, par ticularly those elements in group 1 of group 3. The characteristics of sodium, however, are generally more favorable than those of other elements.

A slightly more complex mode of operation has definite advantages over the relatively simple resonance radiation excitation described above. This mode of operation utilizes enhancement of intensity by collisions of the second kind to enhance the intensity of a particular spectral line from the lamp.

Considerable study has been made of the phenomenon of sensitized fluorescence. Atoms of one kind, excited to a particular electronic level, may, on collision with atoms of a second kind, transfer their excitation energy. It has been shown experimentally and theoretically that the transfer process is most probable if two conditions are fulfilled:

(a) The smaller the energy difference between the levels of interest in the two kinds of atoms, the greater is the collision cross-section for the exchange.

(b) The total electronic angular momentum of the two atoms remains the same before and after the collision (Wigner partial selection rule).

In connection with rule (a), the energy difference must be converted to or from kinetic energy of the atoms. If the energy difference is less than thermal energy KT-0.03 ev.) and if rule (b) is obeyed, the crosssection may be more than 100 times the kinetic theory cross-section. In particular, collisions of the second kind have been observed between metastable Hg (6 P atoms and sodium atoms in a mixed gas. -It will be observed from the diagram of FIG. 2 that the Hg (6 P level falls between the Na (78) and Na (6P) levels and is 0.045 ev. from either. It has been observed that the visible Na (7S 3P) 4751 A. line became as intense as the Na (3P- 3S) 5893 A. line under certain conditions, showing that the bulk of the energy was transferred to the Na (75) level. The intensity enhancement will be about 20 times. It may be expected that transitions from the 6P level will be similarly enhanced.

The proper mixture of Hg in Na amalgam to obtain the necessary pressure of both Na mm. Hg) and Hg (-l.0 mm. Hg) at operating temperature can be obtained from published data or approximately from Raoults law.

From the foregoing explanation, it will be seen that by utilization of collisions of the second kind with a different kind of atom, the efiiciency of the operation by which a greater population of a higher energy level is produced by optical-pumping may be substantially increased with a resulting increase in efiiciency of operation of the light amplifying device.

Nonresonant light amplifier with internal discharge FIG. 3 shows a modification of the nonresonant light amplifier in which the gaseous medium within the cavity is excited directly by application of radio frequency energy rather than being excited by a light source as in FIG. 1 (low frequency energy or a direct current discharge could be used instead where desired).

A cavity 31 is provided having an opening 32 for the transfer of light output to the exterior of the cavity. A

rod of transparent material shown at 34 may be utilized to transmit the light output from the apparatus, or alternatively, windows may be used as illustrated in FIG. 1.

The interior 36 of the cavity is preferably filled with a gaseous medium such as a mixture of mercury and sodium vapors as previously described. The wall 37 of cavity 31 is provided with a reflecting surface 40 such as magnesium oxide.

A reservoir 38 is connected by a conduit 39 to the interior 36 of the cavity 31 in order to provide a gaseous atmosphere of the desired composition and pressure within the cavity 31. A heater 41 is provided for the reservoir 38 and is controlled by a temperature regulator 42 thus providing control of the pressure of the vapors within the cavity. Excessive fluctuation of pressure within the cavity 31 and condensation Within the cavity 31 is prevented by maintaining the cavity 31 in a temperature controlled oven 43.

Energy is supplied to excite the atoms within the cavity 33. by a coil 44 surrounding the cavity and supplied with high frequency excitation which may be of a frequency of approximately megacycles for example.

Thus in the device of FIG. 3 excitation of atoms within cavity 31 is by a radio frequency energy induced discharge rather than by light excitation as in the apparatus of FIG. 1. In other respects the operation of the apparatus of FIG. 3 is substantially similar to that of FIG. 1.

Obviously the discharge within the cavity 31 may be produced in other manners such as by a direct current or low frequency discharge between electrodes within the cavity or by capacitively coupling high frequency electrical energy into the cavity rather than using the inductive coupling illustrated in FIG. 3.

The advantage of producing a discharge within the cavity rather than depending upon absorption of energy from a light source is readily understandable when it is realized that only approximately 20% or less of the light energy directed into the cavity is absorbed to produce useful pumping action. Furthermore, only a limited amount of pumping light can be introduced through a small hole. As the hole is made larger, the loss of light from the cavity becomes substantial. Where the discharge is produced within the cavity to excite the atoms, substantially all of the energy introduced into the cavity is absorbed in the working medium and a large fraction converted to useful output. If an internal discharge is used to excite the atoms, a large amount of pumping power may be coupled into the cavity.

The usefulness of the nonresonant light amplifier is somewhat limited by the large amount of noise present in the output signal. Random fluctuations in frequency or phase of the signal are generated by spontaneous transitions. In particular the usefulness of the nonresonant apparatus as an amplifier (as contrasted with an oscillator) is limited by this background of random spontaneous emission giving rise to a noise bandwidth of approximately 1000 megacycles (the Doppler width of the spectral line). The approximate equivalent noise temperature of the nonresonant light amplifier at the center of the visible spectrum is 30,000 K.

On the other hand, the nonresonant light amplifier, operating as an oscillator, emits an optical line up to 50 times narrower and 3,000 times as intense as the weak spontaneous emission background on which it is superimposed.

Furthermore, the nonresonant light amplifier, in spite of short term fluctuations in frequency, has a long term average frequency which is very constant. Thus by averaging the frequency over a finite period of time a light frequency standard may be obtained having a degree of accuracy comparable with that of any known frequency standard. Such a standard is useful not only in the measurement of time but also in the measurement of distance by interferometric techniques as will be understood by the description of systems for these purposes described hereinafter.

Resonant light amplifier The previously described light amplifier(s) of FIGS. 1 and 3 are termed a nonresonant light amplifier(s because the frequency of the light output, while relatively constant, is not to any substantial extent dependent upon the dimensions of the cavity within which the oscillation is generated.

An alternative form of the light amplifier will now be described in which the resonant frequency of the device is highly dependent upon the dimensions of the cavity. The resonant light amplifier also differs in other important respects, but the resonant characteristic of the device is utilized as a convenient way of distinguishing it from the previously described nonresonant light amplifier.

In FIG. 4 there is shown an elongated cavity 51 enclosed at the ends by end portions 52 and 53. As was the case with the nonresonant apparatus, a reservoir 54 is provided for supplying a gaseous atmosphere to the interior of the cavity. A heater 55 illustrated as a heating coil is controlled by a temperature regulator 56 to insure control of the pressure within the cavity 51.

As in the case of the light amplifiers of FIGS. 1 and 3, an oven 57 may be provided to enclose a portion of the apparatus to maintain it at a temperature higher than that of the reservoir 54 thus preventing condensation within the cavity 51 and allowing closer control of the pressure of a gaseous atmosphere.

From the foregoing explanation, it will be seen that the interior 58 of the cavity 51 is, in this form of the invention, supplied with a gaseous medium, the pressure of which can be controlled by means of the temperature regulator 56. The gaseous medium within the cavity 51 in FIG. 4 will be considered to be sodium, although as previously explained, other mediums may be used.

Excitation for the medium within the cavity 51 is provided by a cylindrical gas discharge lamp 59 surrounding the cavity 51. The gas discharge lamp 59 is preferably a sodium vapor lamp filled with a suitable gas or a combination of gases such as sodium and argon.

The outer wall 61 of the lamp 59 may be provided with a reflecting surface such as magnesium oxide to conserve light, whereas the inner wall 62 of the lamp 59 is preferably highly transparent to the desired spectral com-ponents of the light produced by the lamp.

The wall of the cavity 51 is also preferably highly transparent to this light. It is obvious that if desired a single wall may be provided between the interior 60 of the lamp 59 and the interior 58 of the cavity 51, thus making these two portions of the device as one integral element. The wall 62 may be formed of a material acting as an optical filter, if desired, thus discriminating against certain components of the light from lamp 59 which are not desired. The lamp 59 may also be provided with a reservoir 63, a heater 64 and a temperature regulator 65 in a manner similar to that provided for the cavity 51 so that the pressure within the discharge lamp may be independently controlled by means of the temperature regulator 65. It will be noted that the oven 57 also maintains the interior of the discharge lamp 59 at a higher temperature than that of the reservoir 63.

Electrodes 66 are provided in the lamp 59 and are supplied by power from a supply 67 through leads 68. The nature of the electrical excitation of the lamp 59 may be selected for the best results in a particular application and may be, for example, direct current, alternating current, or

high frequency radio frequency excitation, etc.

It has previously been noted that it is desirable to provide means to confine and retain the light energy within the cavity in order that a number of emissions of light energy will be stimulated and the intensity of the light will in that the reflectors are specular reflectors rather than diffuse reflectors as used in the nonresonant cavity.

The reflectors in the cavity 51 comprise prisms 69 and 71. Mirrors may be used as reflectors in the cavity 51 but in many instances prisms are preferable due to the requirement for an extraordinary high degree of planarity and parallelism when plane mirrors are used and which requirement is significantly reduced by the use of prisms.

Thus the use of prisms is a feature of the invention of great practical importance. Prisms 69 and 71 are illustrated as triangular right-angled prisms. That is, the faces 73 of prism 71 are at right angles to each other as are the faces 72 of prism 69 (one of the faces 72 is not visible due to the orientation of a prism 69). Such 90 prisms can be ground with a high degree of accuracy. Assuming that the prisms are so ground, it is known that light rays entering the faces or 74 of the prisms 71 and 69, respectively, are returned almost exactly in the direction from which they originated for a substantial range of angles of incidence with the front face (75 of the prism 71, for example). Furthermore, the effective pathlength for rays entering the face 75 is substantially the same over the surface of the face even though the angle at which the rays strike the face 75 is not exactly The prisms 69 and 71 are preferably oriented so that their rear roof) edges joining the diagonal faces are at 90. That is, in FIG. 4 the edges joining the reflecting faces of the prism 69 are vertical while the edges joining the reflecting faces of prism 71 are horizontal.

Accordingly, with face 75 nearly perpendicular to the direction of impinging light rays, the prism 71 may be rotated several degrees about a horizontal axis extending into the paper without causing an appreciable change in the direction of reflection. The prism 69 can be rotated several degrees about a vertical axis without causing an appreciable change in the direction of the reflected rays. As a result, the placement of the two prisms 69 and 71 is not critical with respect to rotation about either of the orthogonal axes parallel to the rear edges of the prisms. As a result, the prisms 69 and 71 once ground to the tolerance required as regards the planarity and angular relationship of the various faces can be placed within the cavity 51 without any highly critical requirements of parallelism as regards the faces 74 and 75.

There are several alternative ways to reduce the criticality of the angular positioning of the reflectors. For example, one may replace the prism 71 with a corner reflector with three mutually perpendicular planar surfaces (which may also be a prism) and replace the prism 69 with a plane mirror. The mirror may be a low loss multilayer reflector which selectively reflects only light of the desired wavelength.

The advantage of utilizing prisms rather than mirrors may be appreciated by consideration of the general mode of operation of the resonant light amplifier. It is desired that the light rays traverse the distance back and forth between the reflecting beams a considerable number of times. If the optical pathlength over each circuit of the two reflecting means, and in fact, if the path circuit over a plurality of circuits of the reflecting means is not the same for each and every portion of the reflecting surface within the accuracy of a fraction of the wavelength, interference will be produced and a resonant nature of the system will be diminished or destroyed.

It is likely that one limit of the efficiency of the system will be the tolerances to which flat optical surfaces may be produced. It may be impossible to obtain a surface with a closer tolerance of flatness than approximately onefiftieth of a wavelength as a practical matter. This, of course, will limit the efficiency of resonant light amplifiers utilizing prisms as well as the resonant light amplifier utilizing flat mirrors. In the case of the mirrors, however, it would also be necessary to place and retain the mirrors in respective ends of the cavity (which may be separated in a typical case by 30 centimeters) in parallel relationship with a tolerance of one-fiftieth of a wavelength, approximately. This can likely be achieved although it would necessarily involve a phenomenal degree of precision and expensive techniques that would go with such a precise operation. Furthermore, the completed device would be highly sensitive to disturbances and vibration of all types including physical accelerations, changes in temperature, etc.

The prisms 69 and 71 are preferably provided with non-reflective coating on their front faces 74 and 75, as light reflected from these faces will generally be lost due to being out of phase or slightly misdirected and will not add coherently to the main standing wave in the cavity.

The faces 73 of the prism 71 would normally be substantially 100% reflective. An output from the cavity (or in the case of an amplifier operation, an input as well) may be provided through one or both of the faces 72 of the prism 69. The face 72 may be rendered partially transmissive by placing on or near the face a material which has an index of refraction which does not differ from the index of refraction of the prism sufiiciently to provide total internal reflection. By this means, any desired portion of the light impinging on one or both faces 72 may be transmitted to the outside of the cavity. Conversely, if the apparatus is to be used as an amplifier, thus necessitating an input, the same path or a similar path may be used for the input to the light amplifier.

In the case of an amplification operation as contrasted to an oscillator operation, there will generally be a loss of energy involved in transmitting the input signal into the cavity and transmitting the output signal out of the cavity. Obviously any normal type of transmission path for light energy into the cavity will also provide a path for the same kind of light energy out of the cavity. One may expect a loss on the order of 50% in this operation, but this will not be serious in view of the overall gain produced 'by the light amplifier. Such a problem need not arise in the case of a light oscillator as no input signal is required due to the fact that oscillations are built up from ever-present random fluctuations as is the case with other types of oscillator devices.

Operation of resonant light amplifier As previously explained, the induced emission from atoms is coherent with the inducing radiation. That is, it has the same phase, frequency and polarization. If many atoms over the breadth of the inducing wave are emitting, the emitted radiation will also be a substantially plane wave with the same propagation vector except for small diffraction effects. With this understanding it will be seen that the resonant light amplifier of FIG. 4, although it has only small reflecting surfaces compared with its total cavity internal area, effectively confines the amplifying operation due to the fact that only light energy within a very narrow range of frequency and propagation direction is amplified and this energy has a direction of propagation vector such that it is substantially contained between the two reflecting surfaces.

There will be slight losses of energy off the edge of the reflectors due to slight discrepancies in the angle of propagation of the rays being amplified. This slight energy loss will not be suflicient in a well designed apparatus to prevent proper operation of the device.

Within its frequency an angular limits, determined by the dimensions and loss coeflicient on reflection, the resonant light amplifier will amplify plane waves continuously variable in direction and frequency.

If the input wave is plane, the output wave is almost but not exactly plane. The finite size of a wavelength, 7\, allows the wave front to spread as it travels. At great distances from a circular end-plate, the wave front, instead of remaining a circle of constant diameter, exhibits the Fraunhofer diffraction pattern of intensities. In

this pattern approximately 98% of the light falls in a central spot of angular radius more than half the light falls in a cone of half this angular radius. If the wave is focused on a nearby plane, one observes the same pattern instead of a point. The Rayleigh criterion for angular resolution of two plane waves focused in a telescope is that the waves shall make an angle with each other equal to A9. That is, the maximum of one falls on the first dark ring of the other pattern. Thus plane waves from different points of a distant object could be amplified coherently by the resonant light amplifier and then focused on a screen or the face of a television camera tube. The resulting image could be scanned or otherwise used.

If a plane wave passes through a circular aperture, then at nearby distances the wave starts to spread and forms the Fresnel diffraction pattern.

Thus as a plane wave reflects back and forth inside the tube, light dribbles out of the cylindrical space between the reflectors. The fraction of light lost by this mechanism in travelling a distance l=L/a is very approximately given by =fraot.lost 1;

where L/a is the mean distance travelled by a photon before it is lost at a reflector.

If 1 then the effective loss on reflection, a, will be appreciably increased. This puts a lower limit on the radius of the reflector. If

L=100 cm. A=5 X 10 cm. a=.05

then Dii-03 which is about as high as desirable.

Amplifier with brewster angle prisms Another form of resonant light amplifier is shown in FIG. 5, comprising a cavity 81 filled with a working medium 82, of any of the types described herein.

A prism 83 is provided in the cavity 81. This prism comprises two portions, 83a and 83b.

The portion 83a of prism 83 is a 90 triangular prism, the triangular faces of which are not visible in FIG. 5. One reflecting face 84 of the prism 83 is shown in FIG. 5; the other reflecting face is on the reverse side of the prism and is not visible. The edge of the front face 85a of the prism 83 is also shown in FIG. 5.

In practice, it may be desirable to form the prism 3 3 from one solid piece of transparent material in which case there will be no actual face 85:; as indicated. However, the overall effect will be the same.

A second portion 83b of the prism 83 is also formed in the shape of a triangular prism; in this case a triangular face is visible in FIG. 5. Although the portion 83b of the prism -83 is shown as a right triangular prism in FIG. 5, there is no necessity for the right angle corner of the prism to be accurately formed, and in fact this angle need not be a right angle.

The front face 855 of the prism 83 is disposed at an angle to the longitudinal axis of the cavity 81 which is approximately equal to the cavity 81 which is approxi mately equal to the Brewsters angle for the prism 83. The Brewsters angle is that angle at which a wave polarized parallel to the plane of incidence is wholly transmitted (with no reflection) and a Wave polarized at right angles to the plane of incidence is substantially totally reflected.

The prism 83 is preferably shaped so that rays incident upon the face 85b at the Brewsters angle (indicated by the angle 0,, in FIG. 5) are refracted to be approximately perpendicular to the face 85a of prism 83a (or in the t5 event that portions 83a and 83b are formed from a single unitary piece of transparent material, then the plane separating these portions).

A second prism 86 is located at the other end of the cavity 8-1. The prism 86 is a right triangular prism oriented so that one triangular face is visible in FIG. 5. The edges of three rectangular faces of the prism 86 are visible in FIG. 5, namely those of reflecting faces 87a and 87b and of front face 88.

Front face 88 is also disposed at an angle with the longitudinal axis of the cavity 81 equal to Brewsters angle for the material of which prism 86 is formed. This angle is indicated by 0 Prism 86 is preferably cut so that rays incident upon face 88 at Brewsters angle are refracted to strike reflecting faces 87a and 87b at approximately an angle of 45.

Due to the angle of incidence of the rays on the faces 85b and 88 of the prisms 83 and 86, respectively, in FIG. 5, the optical system of FIG. 5 functions not only to provide a closed path for light rays, but also to reject the light having other than a particular polarization. At the same time the problem of partial reflection of light from the front face of the prism is substantially eliminated by the orientation of these faces.

A window 89 is provided in the cavity 81 arranged to receive light reflected from the front face 88 of prism 86. From the previous explanation it will be understood that there would normally be no reflection of light generated in the cavity from the face 88, as there would be a closed path (and amplification) provided only for light of a particular polarization and this light would be totally transmitted through face 88. However, in order to provide an output from the cavity 81, the face 88 may be rendered partially reflecting by the addition of a coating, for example. An output may be provided also by setting the prism 86 at an angle differing somewhat from Brewsters angle and thereby causing a small amount of reflection of the polarized light generated within cavity 81. In some instances it may be desired to control the reflectivity from the face 88, which will, among other effects, control the output from the cavity 81; this may be accomplished by rotatably mounting the prism 86 so that the angle of incidence of rays with the front face 83 may be controlled by rotating the prism 86 about an axis perpendicular to the plane of the paper.

From the foregoing explanation it will be understood that FIG. 5 represents a preferred form of optical system which may be utilized in the light amplifying apparatus of FIG. 4 or others of the resonant light amplifiers, and which provides the advantage of substantially eliminating unwanted reflection from the front face of the prism, and at the same time provides a polarized output which is obtained by a filtering action inside the cavity 81. Such filtering action within the closed path inside the cavity is preferred to filtering the output from the light amplifier due to the fact that the approximately 50% power loss resulting from external polarization is substantially eliminated. The optical system of FIG. 5 also allows the reflectivity of the face 88 to be continuously controlled and thus allows controlling of the output from the light amplifier.

In addition to being useful in resonant light amplifiers, the optical system of FIG. 5 is useful in other applications where a light resonator is employed such as in a Fabry-Perot interferometer.

Mirror type resonanl light amplifier FIG. 6 shows an alternative form of resonant light amplifier device comprising a cavity 101 having transparent side walls and enclosed at its ends by flat mirrors 102 and 103.

The interior 104 of the cavity 101 is filled with a sensitized working medium such as sodium vapor. Placed around the cavity 101 is a concentric cylindrical discharge tube 105. The surface 106 may be provided with a reflective coating to conserve light while the inner wall litssasia bra 107 of the discharge tube 105 is transparent to the desired component of the light produced in the discharge tube.

Electrodes 103 are provided in the discharge tube 105 which are supplied with power from a power supply 109 through leads 111.

In the form of apparatus shown in FIG. 6 the medium in the gas discharge tube 105 is a mixture of sodium and mercury. As previously explained, such a mixture provides an enhancement of a desired spectral line by collisions of the second kind. This brings about a considerably increased intensity of the desired spectral line in the lamp and increases the optical pumping power which creates the desired population distribution in the energy levels of the atoms of sodium in the interior 104 in the cavity 101 conducive to stimulated emission of light radiation.

In FIG. 6 the reservoirs, ovens, and auxiliary equipment for maintaining the proper atmosphere in the discharge tube 105 and in the cavity 101 have been omitted for simplicity. Such elements may be provided for the apparatus of FIG. 6 in accordance with other figures of the drawings or any other suitable means for maintaining the appropriate atmosphere may be utilized.

The mirrors 102 and 103 may be metallized or multilayered iuterference reflectors. The latter are almost lossless tie, the transmission plus the reflection equals approximately 100%). Interference reflectors may have a very high reflectance, for a given wavelength, depending on the number of layers. A practical achievement is 98% in the visible for a 7-layer reflector. Flats with a closer tolerance than approximately h are not currently available so if a resonant system is desired and more accurate flats are not available, higher reflectance would not be useful. An additional advantage of interference reflectors is that photons from other than the desired transition would not be reflected (due to frequency selectively), and hence, undesired stimulated transitions would be prevented.

It is clear from FIG. 6 that a plane wave travelling in a direction other than 90 to the mirror surface will walk off the edge and lose energy at a rate faster than the normal wave. The lateral displacement per reflection is where a, is the loss at the reflector due to absorption and diffraction, or the rate of energy loss is E dt 1, L 212} As a increases, the gain of the light amplifier will decline proportionately in the range of linear amplification. A practical measure of the limiting angle at which eflective amplification obtains might be that angle for which a 2:: absorption:

the maximum one might conceive would be 0 -0.1 radian, while for the dimensions immediately above,

0 -5 X 10- radian he fact that the loss cocliicient falls off with increasing angle, 0, determines a most important characteristic of the resonant light oscillator output, a very narrow beam.

It can be calculated that virtually the entire output beam will fall within the Farunhofer diffraction pattern for 6:0. It may also be shown that, with P =l watt at xzl the frequency bandwidth of the output beam will be less than 100 cycles/sec. This is residual bandwith due to the noise discussed below.

As pointed out previously, the random-fluctuation spontaneous emission background in the visible will correspond to transitions induced by thermal radiation at a temperature of 30,000" K. However, this is not so high as it first appears, since a resonant light amplifier may discriminate against all signals outside a narrow optical band and against all directions of propagation outside the central Fraunhofer lobe. 7

It can be estimated that the minimum equivalent noise input power in a Fraunhofer lobe is given by:

P zlfi X 10- watts in the visible If the bandwidth, A is limited in a succeeding electronic amplifier, it can be shown that the following expression for this noise holds Thus the minimum noise depends on the square root of the bandwidth but not on the area of the reflectors 102, 103 at the tube ends.

The apparatus of FIG. 6 may be used as an amplifier as distinguished from a self-sustained oscillator by limiting the gain; that is, by limiting the amount of light power introduced from the discharge tube 105 so that a selfsustained oscillation is not produced. A signal may, therefore, be introduced through the mirror 103 as indicated by the arrow 112 as the mirrors 103 and 102 are partially transmitting.

The light ray indicated at 112 will cause stimulated emission of light energy within the cavity 101 which is coherent with the input signal with respect to phase, frequency and direction of propagation. The amplification within the cavity is rather selective with respect to direction of propagation and frequency so that only a relatively small range exists with respect to these two parameters within which an input wave will be amplified in the device.

The output from the light amplifier will be transmitted through mirrors 103 and 102 as indicated by the arrows 113 and 115. Either or both of these outputs may be utilized, depending upon the particular application or system in which the light amplifier is used.

As in the case of previously discussed light amplifier devices, the device of FIG. 6 may also be utilized as an oscillator simply by increasing the efiiciency of the process or otherwise increasing the gain of the amplifier to the point where self-sustained oscillations are produced. In certain applications it may be desirable to utilize the same apparatus as both an oscillator and an amplifier, on a time sharing basis, for example. This may be accomplished, for example, by periodically increasing the light energy produced by the discharge tube 105 to momentarily produce self-sustained oscillations. It should be understood that the optical system can be replaced by other optical systems such as those illustrated in FIG. 4 and FIG. 5, and also that the exciting process utilized in FIG. 6 may be replaced by other exciting processes previously described.

It should be noted that the apparatus of FIG. 6 does not differ greatly from the nonresonant cylindrical amplifier previously described, and the resonant apparatus in FIG. 6 could be converted to a nonresonant amplifier 18 by substitution of diffuse reflectors for the mirrors 102 and 103.

Resonant light amplifier with internal discharge The technique of exciting the atoms within the cavity may be applied to resonant light amplifiers as illustrated in FIG. 7. An elongated cavity 121 is provided having enclosed ends 122 and 123. Desirably at least one of the ends such as 123 is transparent to light of the frequency for which the amplifier is designed. In the case of the cavity 121 the side walls need not be transparent as in previously described cavities because there is no necessity for introducing light energy through the walls, as different means of excitation are used.

The interior 124 of the cavity 121 is provided with a gaseous atmosphere which may be supplied from a reservoir 125 heated by a heating coil 126 controlled by a temperature regulator 127. As previously described in the explanation of other forms of the apparatus, the elements 125, 126 and 127 in conjunction with a temperature control oven 130 surrounding the cavity 121 assure that the pressure of the medium within the cavity is maintained at the proper value.

Preferably the interior 124 of the cavity 121 is filled with a gaseous atmosphere comprising a mixture of sodium and mercury, or with some other mixture of elements by means of which the efficiency of exciting the working atoms to a desired eenrgy level is enhanced by reason of collisions of the second kind. The general theory by which more desirable population distribution among the energy levels of one element is produced by collisions of the second kind with another element has previously been explained and will not be repeated here. The optical system of the device of FIG. 7 is similar to those previously described in that it comprises two prisms 131 and 134 having 90 angled faces 132 and 135, respectively, and front faces 133 and 136. As previously explained, the front faces 133 and 136 are preferably provided with a low-reflection coating so that substantially all of the reflection is by internal reflection from the faces 132 and 135.

At least one of the faces may be provided with a coating of a medium having an index of refraction intermediate between that of the prism 134 and the atmosphere in which it resides thus preventing total reflection at the face 135 and allowing transmission of light generated within the cavity 121 through the prism 134. It may be noted at this point that light passing through the prism 134 may be divided into several beams by reflection and refraction. In some cases this may be desirable, but in the event that it is desired to direct substantially all of the output beam in one direction, additional prisms can be provided for combining the various output beams by reflection or refraction to be directed substantially in the same direction.

An optical filter 137 may be included in the light path between the prisms 131 and 134 for the purpose of discriminating against light of a frequency other than that selected for the operation of the light amplifier. Particularly when such amplifier is operated as an oscillator, there may be an atomic transition of higher probability than that generating the desired light frequency. Such a transition would generate an oscillation at a lower input power than required for the desired oscillation. Such parasitic oscillations must be suppressed. Generally, oscillation in several modes simultaneously will not occur,

, except as transients, and would in any case be undesirable.

filter within the closed path in the light amplifying device will introduce losses for light of all that the polarity for which the filter is transparent. These losses will greatly degenerate the amplification for other than the desired polarity of light and in an oscillator type device will prevent the generation of self-sustained oscillations except for light of the desired polarity.

The operation of a resonant light amplifier with .a polarizing filter is therefore similar to that described for the apparatus of FIG. which also includes polarizing means within the closed light path in the light amplifier. As in the case of the apparatus of FIG. 5, polarization within the light amplifier device has advantages over polarizing the light output from the device, in that loss of substantially half of the power, as would occur in polarization of the output, is substantially avoided. Furthermore, in some instances the light intensity may be sufficiently high in the output so that polarization of the light would present a heat dissipation problem. When the light is polarized within the light amplifier device, it is polarized before light of the unwanted polarization has a chance to build up in intensity and accordingly the heat or energy dissipation problem is substantially reduced.

It will be noted that in the apparatus of FIG. 7, no separate gas discharge lamp is provided for light excitation of the medium within a cavity. Instead a discharge is excited directly within the cavity 121 by means of electrodes 120 connected to a power supply 128 through leads 129.

Electrodes 120 in FIG. 7 are arranged inside the cavity 121 and may be energized to provide either a direct current or low frequency discharge. It is not necessary however, that the electrodes be within the cavity to produce a discharge within the cavity. For example, electrodes outside the cavity formed of aluminum foil or the like may be utilized to induce an R.F. electric discharge in the cavity 121.

As previously explained, producing the discharge within the cavity provides the immediate advantage of increased energy transfer into the medium within the cavity. Virtually complete transfer of energy into the medium can be accomplished by this means whereas excitation by a light source is limited to approximately absorp tion of the light power introduced into the cavity. Other advantages of excitation by discharge within the cavity also arise.

The higher S-levels of sodium cannot be excited by resonance radiation since radiative transitions between states of the same orbital angular momentum are forbidden by the electric dipole selection rule. However, they can be excited by collisions of the second kind in a discharge within the cavity. In the case of excitation of Na (7 5 by collisions with Hg (6 P metastables. the Wigner partial selection rule relating to collisions of the second kind is satisfied and the collision cross-country may be expected to be about equal to that for excitation to the 6 P Na level. The enhancement of population should be at least 100 times that in a sodium-argon discharge. One may expect to exceed the condition for oscillation in this case by a factor of 20 (for a tube 1 cm. diameter and 100 cm. length).

It can be seen from the diagram in FIG. 2 that, in addi tion to light amplifier oscillation via the l2,920 A. (7S 4P) transition, there is the possibility of light amplifier oscillation in the visible 4751 A. (7S- 3P) transition in sodium. Although the Einstein B coefficient is larger for the former than for the latter transition, the (7S 3P) or any other particular transition can be favored by inserting filters to absorb all other wavelengths or by using multiple layer reflectors (which reflect only a narrow band of wavelengths). Thus only for the selected transition will losses be small enough to permit the buildup of oscillation.

The pressure of sodium, and hence the operatin temperature of the light amplifier tube, may be smaller since the density of Na atoms need only be large enough to ensure an adequate number of collisions/sec. with metastable Hg atoms mmm. Hg) rather than enough to absorb the (6P 3S) resonance radiation.

This lower pressure can he arrived at as follows. It is known that under the usual conditions in a plasma the populations of metastable states are in thermal equilibrium at the electron temperature (of the order of 5000 C. for a Na-Hg discharge). Hence, the population of Hg (6 P metastables will be -10 of Hg (6 8 the ground level. in a discharge of a few amperes. In a total pressure of l0 mm. Hg of Hg, the metastable partial pressure would be 10' of Hg or a density, N-l0 /cm.

The rate at which a sodium atom is excited to the 75 or 6? levels must correspond roughly with rates of quenching by collisions of the second kind in the case of N and other such gases. For a partial pressure of Hg (6 P P-l0 mm. Hg, this rate of Na excitation would be q/NaglO see. For a sodium pressure, P-10- mm. Hg or N -10 /cm. the total excitation ratio -l0 /cm. sec. For a light amplifier tube of volume cmfi, this gives 10 excitations/see, or about 30 times the calculated necessary rate. If this power is substantially used to generate coherent photons by light amplifier action, a beam power of will be produced.

Conversely the rate at which Hg metastables are attacked by Na atoms will be -l0 sec for a metastable life-time T-lO- sec. This is at least 100 times shorter than the diffusion time to the walls and hence there will be practically no useless loss of metastables via this mechanism. Thus the efficiency of conversion will be relatively high (of the order of 10 percent of input power). The discharge current must, of course, be sufficient to keep up the equilibrium population of metastables. A discharge power, P=3 amps 0.3 volt/cm. 100 cm. =l00 watts should normally be adequate.

Other atomic levels excitable in a discharge In addition to the excitation of the Na (6P and 75) levels by collisions of the second kind with Hg (6 P metastables in a discharge within the light amplifier tube, a number of other energy levels are prospectively suitable for excitation in a discharge. These other levels could not normally be expected to be excited by radiation falling on the tube from outside, either because radiationinduced transitions from the ground level are forbidden,, or because the excitin radiation falls too far in the ultraviolet to pass through even the most transmissive medium such as a quartz tube wall.

A list of metastable levels which may be used to excite levels of nearly the same energy in other atoms by collisions of the second kind is given in Table I. These levels are long-lived because the electric dipole selection rules prohibit decay via this rapid radiative process to any lower level. Metastable levels are listed only for atoms which normally form a monotomic gas (uncombined in molecules) though others could possibly be utilized. Table I is not complete but contains levels most likely to be of practical use. The alkaline earth elements and Zn, Cd are not easily vaporized.

The processes listed below are not analyzed in great detail. but are listed as likely to be useful in particular cases or applications where particular frequencies or other characteristics are desired.

TABLE I Element Metastable Energy Above Ionization Levels Ground Potential Sn (v.) 2 7. 3 Pb 7. 4 Be, Mg, Ca, Sr, (Ba)..- n Pz, n Po Zn 3P3, n PO 4 9.39 11 1 2, N3P0 37 73 9. 00

Light amplifier action in a helium discharge The lowest non-metastable level of He is so high (Z'P at 21.1 volts) compared to the spacing of the next higher level (3'5 at 22.9 volts) and compared to the ionization potential (24.58 volts), that two unusual consequences are true.

22 He discharge will be desirable for use in a light amplifier device according to the present invention. A similar trapped photon situation exists for the Na(7S 3P) transition.

Other cases of excitation by collisions of the second kind In addition to the selective excitation of Na(7S or 6P) by collision with metastable Hg(6 P atoms in a discharge, there are listed in Table II other atomic metals which may be excited by collision with metastables. In each case higher levels of the Working element fall near metastable levels of a possible carrier gas. In some cases such sensitized fluorescence has already been observed by experimenters. For a fuller understanding of Table II, refer to Table I.

It is obvious that light amplifiers in difi'erent frequency ranges will be desired for various difierent applications and thus this characteristic of the type of discharge utilized, as well as others of its characteristics in addition to its inherent etliciency, will be considered in selecting the medium utilized in a light amplifier.

TABLE II.SOME COLLISIONS OF THE SECOND KIND Firstly, the decay rate E ElKT 'IL 11' [ATP So the ratio of numbers of electrons having energies E and E1 IS The ratio of excitation rates to the 3'5 and 2'P levels by electron collisions will be roughly equal to dn /dn Since E(3'S )E('2'P )=l.7 el ctron-volts and KT-2.5 e.v., it will be seen that the rate of excitation by electrons is almost the same for both levels, while the rate of decay from 2P to the ground level is almost 1,000 times greater. One may, therefore, expect an excess population in 3'8 over that in 2P This does not take account of factors which will tend to raise the population of 2P Firstly, most of the atoms excited to higher levels must decay via the ZP level.

Secondly, the photons emitted during the processes will be trapped in the gas and re-excite atoms to 2P However, it appears that in at least some instances a pure Light amplifier transitions from metastable to ground level In all the transitions discussed so far, the spontaneous decay rate of the excited level was lO /sec., characteristic of allowed electric dipole radiative transitions. Since the excitation rate is of the order of 1O /sec. even using the efficient and selective method of collisions of the second kind, it is evident that the excited level population could not be made greater than the ground level. Thus light amplifier action or emission by mutually-induced transitions could occur only to an intermediate level whose population was kept lower by an even faster rate of spontaneous decay.

It is also possible to depopulate a longer lived lower level by collisions of the second kind, to accomplish the same result.

This situation is in contrast to that of the Maser. For levels spaced only by E=hv where 11 is a microwave frequency, the thermal equilibrium population of the excited level is high and the lifetime is long. Therefore, the population of the excited level is easily maintained above that of the ground level.

In the case of levels high above ground (optical transitions), the initial populations will always be low. But if the levels are long-lived (metastable), it may be possible to populate them at a rate greater than the natural decay rate or relaxation rate, and hence to maintain a higher population of atoms in the higher state.

Many of the elements which exist as single atoms in the gaseous state do not have high vapor pressures at convenient temperatures. Therefore, those which may possess metastable states are not useful in exciting other atoms by collisions of the second kind. Nevertheless, it may be possible to use such atoms as working atoms as described above. For this purpose the pressure may be much lower. Most of these metastable levels are listed in Table I.

As an example, consider the case of zinc. The metastable 4 P levels lie about 4 electron-volts above ground as shown in FIG. 14. These levels do not lie close to any other metastable levels and so cannot be excited directly by collisions of the second kind. However, higher, nonmetastable levels of Zn, can be excited by collisions with metastable krypton and xeron. From these levels the Zn atoms decay rapidly to the metastable levels as well as the ground level.

The rate of decay of Zn (4 P 4S by emission of the ultraviolet photon, ,\=3076 A., is A =l l0 /sec. It has already been mentioned that rates of collision excitation somewhat greater than this may be achievable. Therefore, it should be possible to generate light amplifier transitions directly to the ground level with zinc. One must have @811 ex oo a Substitution of values gives N 5 l0 /cm. If the atoms are shared by collision among the three metastable levels, the required zinc pressure is only 2X10" mm. Hg, which is the vapor pressure at about 200 C.

In the ease of thallium, most of the atoms may be pumped into the much longer lived 6 P metastable level by a similar indirect mechanism: either collisions of the second kind with Hg metastables or absorption of the 3776 A. resonance radiation with subsequent decay to 6 P In this case, since the density must be quite high: N-l0 atoms/cm. or Pz3 l0- mm. Hg at a temperature, T 600 C.

Excitation by coincident spectral lines i Previously, the excitation of atoms by resonance radiation was discussed. The emitted spectral lines from a lamp of the same substance necessarily coincide with frequencies absorbed most strongly by the same type atoms in the light amplifier. However, as pointed out in the discussion of sodium excitation, the intensities of lines emitted from the lamp during decay of higher states are quite weak. It was also pointed out that the intensity of certain of these higher resonance lines could be enhanced by collisions of the second kind with metastable atoms. Another way of Obtaining strong excitation to higher electronic levels is by accidentally coincident bright emission lines from another atom.

The chance coincidence of two appropriate atomic lines is small. There is room for some 300,000 spectral lines of Doppler width with only slight overlap throughout the visible and near ultraviolet range. There are at most 1,000 useful resonance transitions in convenient atoms and approximately bright atomic lines with which to excite them. Thus, there is about a 10% chance of one good coincidence. At least three moderately close coincidences are known, as shown in Table III.

He aw- 2 5 3889 A.-Cs(8 P 6 S )3889 A. Zn 3303.7 A.-Na(4 P 3 S )3303 A. A 8521.4 A.-cs 6 P 6 s 8521.2 A.

However, in none of these cases is the overlap good enough for high excitation efficiency. On the other hand numerous examples of the excitation of molecules by coincident atomic lines have been observed.

Information is scarce on fluorescence of molecules containing more than two atoms. Therefore, only diatomic molecules are considered herein.

Each electronic level in a diatomic molecule is split into approximately 50 vibrational levels and each vibration level into approximately 200 rotational levels.

Therefore, we may expect more than 100,000 absorption transitions from populated levels in every molecule on the average. As expected, there is generally at least one coincidence of a bright atomic spectral line with some resonance transition of a given molecule. By the same token the emission from a discharge in a molecular gas is divided into many weak lines. These cannot be excited by an external lamp conveniently.

Materials which transmit U.V. radiation below 2,000 A. are not available. Therefore, the light amplifier process previously described cannot be used, i.e. excitation to a high electronic level with light amplifier emission to an intermediate level Whose population is kept low by rapid spontaneous decay to a ground level. Instead the properties of molecules require and permit another mechanism for keeping the lower level population lower than some higher level population. This mechanism is relaxation of the lower level population by collisions of the second kind.

To exemplify the whole process, the molecule I is considered (see FIG. 8).

The first member of the sodium principle series at 5893 A. ljsee FIG. 2) coincides with one of the numerous absorption lines of the iodine molecule. The transition in question is from a rotational sublevel of the v=2 vibrational level of the ground electronic state (e'g+) up to the 1:30, 11:17 sublevel of the (311' state. The v=2 of te'g levels are well populated in thermal equilibrium at room temperature (see lower right corner of FIG. 8), while v=7 of (eg+) has less than 1% of the population and v=17 of (31 has none.

A 1 cm. thick layer of I vapor at a few mm. Hg pressure absorbs most of the Na light and raises I molecules to the upper level at a rate avail (It hi1 In the absence of light amplifier action, the atoms decay at a rate hi'Yc-i 6 (all other states where =spontaneous radiative decay rate,

n =population of atoms in the higher energy state, and

y =rate of removal by relaxation collisons with other I molecules quenching collisions). The cross-section for these collisions is very high since many I states are closely spaced in energy. About 5% of the molecules decay to 11:7 of (e'g Then, by the same method of decay as that first described herein, the dynamic equilibrium rates of population change are It is to be noted that atoms can be removed from v=7 of (E'g+) only by relaxation collisions to other sublevels of the ground electronic state. Then if the 1 pressure {-S mm. Hg) is such that IeEA QZO A (he I) -l0 /sec. then which is necessary for light amplifier action.

The further analysis is quite similar to that for the Na(6P- 48) light amplifier transition. The values involved are not much different and so for a light amplifier tube 1 cm. diameter x cm. long, the required Na (5893 A.) intensity from a discharge lamp arranged as a jacket is [:10- watts/em. steradian 

